When the rook pawn reaches the sixth, the defender must immediately bring the rook to the sixth rank (or start checking from the side) before the attacking king defends the pawn.
Here the Vancura set-up has not been reached yet. White's king on c5 is already close to the pawn, and Black must act at once; a single slow move loses.
Common mistakes
Ra4Too slow. White plays Kb5 and the rook is driven off while the king defends the pawn; Vancura cannot be reached in time.
Rf1White replies a7! and Kb6; the rook never gets behind the pawn safely and the king supports promotion.
Kf7Leaves the safe square: Rh8! and the threats of a7 and Rh7+ decide.